X-ray backscatter imaging relies on scanning an object with a well-collimated beam, typically referred to as “pencil beam”. Several approaches for forming the collimated scanning beam have been suggested. Commonly, beam formation and steering relies on an aperture moving in front of a stationary X-ray tube. In most cases the radiation from an X-ray tube is first collimated into a fan beam by a stationary collimator. Then, a moving part with an opening forms a scanning beam. This moving part can be, for example, a rotating disk with radial slits, or a wheel with openings at the perimeter. The rotating disk covers the fan beam and the scanning beam is formed by the radiation emitted through the slits traversing the length of the fan beam opening. This approach is illustrated, e.g., in the U.S. Pat. No. 3,780,291 (to Stein and Swift). In the case of a rotating wheel, a wheel with radial bores spins around the X-ray source. If the source is placed at the center of the wheel (or hub), the scanning beam is emitted in radial direction with the angular speed of the wheel. Alternatively, the source may be placed off-center with respect to the rotating wheel, which changes the beam geometry.
In most X-ray tubes, an electron beam impinges upon a stationary target, which, in turn, gives off X-ray radiation produced by stopping the fast electrons, i.e., Bremsstrahlung. Most of the kinetic energy of the electron beam is converted into heat and only a small fraction is given off as X-rays. For imaging purposes, a small electron beam focal spot is desirable, however anode heating limits the acceptable current for a given focal spot size.
To allow smaller focal spots, X-ray tubes 100 have been designed to have rotating anodes, as depicted in FIG. 1. X-ray tube 100 represents a typical design, as produced, for example, by Varian Medical Systems. Rotating anode 102 distributes the heat over a larger area and allows a considerably smaller focal spot 104 of electrons 106 emanating from cathode block 107 than would be possible using a stationary anode. Rotating anode 102 is rotated by rigid coupling to rotor 108 which moves relative to stator 110. X-rays 112 are emitted through exit window 114, and they are subsequently collimated by some external collimating structure.